The present invention relates to an optical refractor for diffusing light, and more particularly an optical refractor which may be used as a transmissive screen of, for example, a color television system of transmissive type.
A transmissive type projection system such as a projective television system, a projective screen display system or an enlarging projection system is widely used today. Generally, it comprises a transmissive type projection screen and a projecting apparatus. The screen is made of transparent material. The projecting apparatus projects an enlarged picture onto one surface of the screen. The picture is then transmitted to the other surface of the screen and observed by viewers.
Various projecting apparatus are known. Among them are an apparatus of a monochromatic image type for projecting a monochromatic picture and an apparatus of a color image type for projecting different color pictures at the same time to form a composite color picture. The latter is used in combination with a color television receiver and simultaneously projects red, green and blue picture, thus forming a composite color picture on a transmissive type projection screen.
The quality of a picture formed on such transmissive projection screen depends largely on the optical characteristics of the screen. Screens of various structures have been invented. One of the well-known structures is a so-called "single surface lenticular lens structure." The screen of this type is made of transparent material. It has on one surface rows of segment lenses which are parallel and extending in vertical direction. Further, light diffusion treatment is applied on said surface of the screen.
The screen of single surface lenticular lens structure is disadvantageous in the following respect. A picture formed on it is less bright at the peripheral portions. Further, when three different color pictures are projected onto the screen in directions which are different a little, the color balance will differ according to the position where a viewer stands to watch the composite color picture. It will be described how this will happen, with reference to FIG. 1.
FIG. 1 is a sectional view of a known screen S of single surface lenticular lens structure. More precisely, FIG. 1 is a sectional view of one of segment lenses of the screen S. When a first light beam A consisting of rays A-1 to A-5 and containing picture information and a second light beam B consisting of rays B-1 to B-5 and containing picture information are projected onto the segment lens, they are refracted on a convex surface 1 of the segment lens, transmitted through the segment lens and emitted from the flat surface 2 of the segment lens.
As evident from FIG. 1, the axis (not shown) of the light beam A does not coincide with the axis (not shown) of the light beam B. Thus, if the screen S is used together with a projecting apparatus of color image type and three primary color light beams are projected onto the convex surface 1, the color balance perceived in a region between the rays A-1 and A-5 will inevitably differ from that perceived in a region between the rays B-1 and B-5.
Moreover, the screen S has another disadvantage. When parallel rays A-1 to A-5 are applied on the screen 2 as shown in FIG. 2, they propagate from the flat surface 2 in different directions in such way that the most intensive ray is emitted in a direction not perpendicular to the flat surface 2. This means that the picture formed on the flat surface 2 is not uniformly bright. Some portions of the picture are bright, and the others are dark.
Another type of transmissive screen is known, which comprises two projection screen members of single surface lenticular lens structure. The screen members are put together with their flat surfaces in mutual contact and with their rows of segment lenses arranged at right angles. The screen, however, has light transmission characteristics which are similar to those of the screen shown in FIGS. 1 and 2. It is not free of nonuniformity of brightness of a resultant picture and difference in color balance, either.
An improved transmissive projection screen is disclosed in Japanese Patent Disclosure (Kokai) No. 51-100723. This screen is of a double lenticular lens structure. That is, convex segment lenses are formed on both surfaces. The screen cannot regulate light distribution in both a horizontal plane and a vertical plane. It regulates light distribution only in one of these planes. To regulate light distribution in both planes, it must be assisted by a lenticular lens and a correction lens such as a Fresnel lens for projecting incident light substantially perpendicular to the screen. In effect, three optical elements, i.e. the screen, the lenticular lens and the correction lens, constitute a transmissive projection screen. The transmissive projection screen is not only complicated and expensive but also generates moire stripes due to interaction of the optical members. This is why the screen disclosed in the Japanese patent publication has not been put to practical use.